The present invention relates to a waveguide-fed microwave system designed particularly for the Ka band. The invention is especially useful in waveguide-fed cavity-backed spiral antenna systems and is therefore described below particularly with respect to this application.
The cavity-backed spiral antenna has been extensively used since its first introduction over 30 years ago. It is a two-dimensional structure fabricated by photo-etching the spiral configuration on a copper-clad laminate. Of the wide variety of spiral shapes it may take, the three which have received widest attention are the logarithmic or equiangular spiral, the Archimedian spiral, and the rectangular counterpart of the Archimedian spiral. The antenna radiates a bi-directional circularly polarized beam normal to the plane of the printed elements when fed from a balanced two wire transmission line. Unidirectional patterns are obtained by mounting the spiral at the mouth of a cylindrical cavity. This type of antenna has been extensively described in the literature; see for example R. Bawer and J. J. Wolfe, "The Spiral Antenna, IRE National Convention Record, Part 1, pp. 84-95 (1960).
Improper excitation of the spiral antenna produces pattern dissymetry, called "boresight error" or "squint", wherein the electrical center of the antenna pattern, and the mechanical axis of symmetry, do not coincide. This is especially troublesome when the antenna is to be used in direction finding applications.
At the Ka band (26.4 to 40 GHz.), it is customary to use waveguide transmission lines. Cavity-backed spiral antennas, on the other hand, are customarily constructed using a coaxial transmission line and a balanced-to-unbalance transformer (balun) to connect to the spiral itself. The above-referenced Bawer and Wolf publication describes on pp. 88-89, some types of baluns which may be used, but points out that "boresight errors in the order of 15.sup.0 for some polarizations are not at all uncommon" (p. 89). The cavity-backed spiral antennas in use at the present time have reduced the "squint" to about 5.sup.0, which is about the best that can be achieved today because of the use of a balun.
Attempts have been made to further reduce the squint caused by baluns, or to eliminate the need of a balun; for example, see T. Thorvaldesen, "Spiral Antenna With Integrated Detector", Electronic Letters, Vol. 16, NOR, pp. 484-486 (June 5, 1980). This publication, after pointing out that much experimental effort has gone into the design of cavities that will damp out the unwanted modes produced in a balun-fed cavity-backed spiral antenna, proposes to eliminate the need for a balun by placing the detecting diode directly on the spiral itself. However, while putting the detector directly at the feed points of the antenna will improve the patterns as compared to connecting a balun to the feed, the problem is that after detection all frequency information is lost. Also lost, therefore, is much of the usefulness of the antenna, which is often used in connection with a frequency scanning receiver.